CIS 632 / EEC 687 Mobile Computing. Mobile Communications (for Dummies) Chansu Yu. Contents. Modulation Propagation Spread spectrum

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1 CIS 632 / EEC 687 Mobile Computing Mobile Communications (for Dummies) Chansu Yu Contents Modulation Propagation Spread spectrum 2 1

2 Digital Communication 1 0 digital signal t Want to transform to since waves: Why? Natural phenomena produce sine waves When a microphone picks up an audible tone, the output is a sine Electromagnetic radiation can be represented as a sine wave 3 Digital Communication - Fourier Transform 1 1 g( t) c 2 n 1 a sin(2 nft ) n n n 1 b cos(2 nft ) 1 100KHz Frequency domain 0 digital signal t 0 50KHz decomposition t 25k 100k f 25KHz Want to send a variety of signals (FM, Wifi, Satellite, Bluetooth, Remote control, etc.) concurrently. This is why we need modulation! 4 2

3 Digital Communication - Modulation Frequency domain FM WiFi 25k 100k f 90M 90M +5k +40k 2.4G 2.4G +25k +100k f Modulation allows us to send a signal over a bandpass frequency range. If every signal gets its own frequency range, then we can transmit multiple signals simultaneously over a single channel, all using different frequency ranges. Another reason to modulate a signal is to allow the use of a smaller antenna. * What 90M or 2.4G is called? 5 Digital Communication Shift Keying : Digital 0/1 Sine Waves Amplitude Shift Keying (ASK): t Frequency Shift Keying (FSK): t Phase Shift Keying (PSK): t 6 3

4 Binary Amplitude Shift Keying (ASK) 7 4-ASK? Frequency Shift Keying (FSK) 8 4-FSK? 4

Phase Shift Keying (PSK) 0 180 0 0 180 (Phase changes are overlaid over the

throughput: Encode 2 or more bits onto one signal element.

5 Phase Shift Keying (PSK) (Phase changes are overlaid over the carrier signal) 9 Constellation Quadrature PSK (QPSK) A t Higher throughput: Encode 2 or more bits onto one signal element. More fragile: Noise makes it more difficult to distinguish between, for example, 11 and

6 Quadrature PSK (QPSK) Two carriers are 90 degree shifted 11 Constellation Quadrature PSK (QPSK) QPSK Offset QPSK 8QPSK Which is more robust 12 to channel error? 6

In-phase signal (for example a cosine waveform) Quadrature phase signal (for example a sine wave) They

7 QAM (Quadrature Amplitude Modulation) In QAM, a finite number of at least two phases, and at least two amplitudes are used. In-phase signal (for example a cosine waveform) Quadrature phase signal (for example a sine wave) They are amplitude modulated with a finite number of amplitudes, and summed. The resulting signal is equivalent to a combination of PSK and ASK. 13 Constellation for 16 QAM /a/b 14 7

8 Bluetooth and Zigbee 15 Contents Modulation Propagation Spread spectrum 16 8

9 Free-Space Propagation Pr P t G G P r : received power r t 2 4 d P t : transmitted power G r, G t : receiver and transmitter antenna gain (=c/f): wave length In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver) Suppose transmitted signal is x, received signal y = h x, where h is proportional to 1/d² Sometime we write path loss in log scale: Lp = 10 log(pt) 10log(Pr) 18 9

10 19 Let s Do the Math Wifi radio Transmit power (Pt) = 28 dbm =??? Watt Distance (d) = 1km (0.6 mile) Pr = Pt*GtGrht 2 hr 2 /Ld 4 Gt=Gr=1, ht=hr=1.5, L=1 Pr = 3.2 x Watt P db = 10 log ( ) 1 P 2 20 dbm = 10 log ( ) P 1 1mW 10

11 21 1x10-12 Watt 2.5x10-12 Watt 2.5x10-12 Watt 4.0x10-12 Watt 22 What does Pr = 3.2 x Watt mean? 11

/2 size of antenna proportional to wavelength for better

12 Real Antennas Real antennas are not isotropic radiators Some simple antennas: quarter wave /4 on car roofs or half wave dipole /2 size of antenna proportional to wavelength for better transmission/receiving /4 /2 Q: Assume frequency 1 Ghz, =? Real Measurements 12

$Signal Propagation Receiving power additionally influenced by shadowing (e.g. through a wall or a door) refraction depending on the density of a medium reflection at large obstacles$

13 Signal Propagation Receiving power additionally influenced by shadowing (e.g. through a wall or a door) refraction depending on the density of a medium reflection at large obstacles scattering at small obstacles diffraction at edges diffraction shadow fading refraction reflection scattering Shadowing Signal strength loss after passing through obstacles Some sample numbers 13

$to reflection, scattering, diffraction Multiple Rates in IEEE 802.$

14 Multipath Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Multiple Rates in IEEE /a/b/g 28 14

15 Trace from 2004 SIGCOMM (CRAWDAD) PHY Packet Formats Always 1Mbps with BPSK modulation Variable (<4KB) synchronization SFD signal service length HEC Payload (MPDU) DSSS PLCP preamble PLCP header Always 1Mbps with GFSK modulation Variable (<4KB) synchronization SFD PLW PSF HEC Payload (MPDU) FHSS PLCP preamble PLCP header * MPDU: MAC Protocol Data Units 30 Data rate for MPDU: Up to 11Mbps (802.11b, DSSS) in steps of 100kbps Up to 2Mbps (802.11, DSSS) in steps of 100kbps Up to 2Mbps (802.11, FHSS) in steps of 0.5Mbps 15

16 Multi-rate Control in WLANs Relatively simple A client node has only one communication partner (access point) Access point Each node can individually optimize its data rate for the link to AP Already being used in commercial products 31 E.g., increase the rate upon consecutive successes; decrease the rate upon consecutive failures ARF (Autorate Fallback) Auto-Rate Fallback (ARF) The first multi-rate algorithm for 1&2Mbps WaveLAN Many commercially available cards support some sort of ARF for automatic rate selection Use a higher rate upon 10 consecutive successful transmissions When the rate is increased, the first transmission must succeed or the rate is immediately decreased Fall back to a lower rate after 2 consecutive transmission failures 32 16

17 Receiver-Based Auto Rate (RBAR) Upon receiving an RTS frame, The receiver estimates the channel quality based on SINR of the received RTS frame and then Determines the best data rate that the transmitter must use The estimated optimal rate is then sent back to the sender piggybacking in the CTS packet A major drawback: Requires incompatible changes to the IEEE standard CTS+Opt.rate ACK 33 OAR & MAD Opportunistic Auto Rate (OAR) protocol Built on a multi-rate algorithm such as ARF or RBAR Although the channel condition fluctuates, it is consistent for the time duration, which is enough to transmit more than one packet When a high-quality channel condition is observed, send multiple back-to-back data packets Medium Access Diversity (MAD) Looks for the receiver whose channel condition is near its peak based on channel probing Group RTS (GRTS) for query (with list of receivers) and CTS s for replies (with channel condition information) The sender then chooses the receiver that can maximally utilize the channel 34 17

18 Other Multi-rate Adaptation Methods Sender-based Auto-Rate Fallback (ARF, 97) Adaptive ARF (AARF, 04) Estimated Rate Fallback (ERF, 05) MADWIFI (04) Adaptive Multi Rate Retry (AMRR, 05) Collision-Aware Rate Adaptation (CARA, 06) Receiver-based Receiver-Based Auto Rate (RBAR, 01) Opportunistic Auto Rate (OAR, 02) Medium Access Diversity (MAD, 05) 35 Contents Modulation Propagation Spread spectrum 36 18

19 37 Why Spread Spectrum? Initially adopted in military applications, for its resistance to jamming and difficulty of interception (seemingly a noise) Adopted recently in CDMA, WiFi & Bluetooth & Zigbee Power Interference 38 19

21 FHSS (Frequency Hopping SS) t b f t User data f f 3 f 2 f 1 t d 41 t t 3 hops/bit, in general, a very short packet/hop or even a bit/multiple hops as in this example With a binary frequency-modulation scheme in which the carrier simply shifts up or down in frequency by about 150 khz to represent, respectively, a 1 or a 0 OFDM (Orthogonal frequency division multiplexing ) In OFDM technology, the bit string to be transmitted is broken down into N (N>1) bit strings. The N bit strings are then transmitted in parallel through N orthogonal sub-channels

22 IEEE /a/b Physical layer a/g 43 Advanced Topics 44 22

23 Physical Layer Watermarking Adjusting phase difference in M-PSK modulation BPSK modulation 0 (0) π (1) 0 (0) 8PSK modulation: add π/4 to embed watermark bit 1 0 (0) π (1) π/4 (?) * Watermarked DSSS (WDSSS) Spreading Correlation Original PN sequence: Two watermark values are embedded Aware receiver records the flipped positions, interprets them into watermark values 23

24 MAC and Collision Collision AP Repeatedly collide CSMA is not perfect with some random jitter Alice Bob 47 ZigZag Decoding: Combating Hidden Terminals in Wireless Networks, S. Gollakota and D. Katabi (MIT), SIGCOMM ZigZag Decoding st collision 2 nd collision 0 AP 1 3 P a 1 3 P a P b P b Alice Bob Can reconstruct both packets P a and P b!! 48 24

Chapter 2: Wireless Transmission. Mobile Communications. Spread spectrum. Multiplexing. Modulation. Frequencies. Antenna. Signals

Mobile Communications Chapter 2: Wireless Transmission Frequencies Multiplexing Signals Spread spectrum Antenna Modulation Signal propagation Cellular systems Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/